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I purchased this car in June 09 with 65000 miles on it for $3300 CAD. I have many service records for the car. Timing belt done around 62000 and gas tank around same time before I purchased it. The car has AC. Car now has 73000 miles on it.

I've invested new tires on it (all season Michelins), a new battery, alternator, car deck and front speakers (infinity 62.9i).

When I drive the car in summer conditions, I hit 264 miles per tank (never get more than 11 gallons in the tank) as I ride the thing to the E light. Winter conditions I understand and agree that the mileage dips. I have spoken to two people who drove either a 2000 Si or a 2002 Si and they both said 372 miles to tank on their manuals.

Is my mileage low for a car of this age in this condition? No lights showing up. I've read O2 sensor could be replaced. Does the engine light automatically light up when it is broken/not functioning properly?

 
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As I just found out with a NEW O2 that was faulty .. it doesn't always light up if it isn't functioning properly... I replaced it Again, and now I am getting 355 to 380 a tank... was getting Very terrible mileage before then. If you know the O2 has never been replaced, it may not hurt to do so, specially with the mileage stacking up on the car.

 
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Thanks for the info about your situation. I will replace the O2 sensor(s) and I believe six generations have two O2 sensors. One monitors the other.

 
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The second one shouldn't have an effect on your fuel mileage/performance. It monitors how well your cat' burns off excess fuel that made it from the combustion chamber. On the other hand, if your second one is throwing a code as well, it may be worth it to check out the cat to see if it is clogged or shot as that Would effect your mileage and performance greatly. Signs of a clogged cat would include sluggishness, taking forever to get up the Rpm band, and craptacular fuel mileage..

 
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yeah the top one on the exhaust manifold is the one that effects your gas mileage. Mine is out right now, but I'm still getting 32 mpg, I am nervous about changing it because the manifold is cracked on the top and bottom of the thread where the o2 sensor is. Also, another sign of a clogged cat, when the car gets up to running temperature you will smell a nasty egg-smell coming from the car, if it's night time, go drive that thing around for 20 minutes or so and look under it, your car will be glowing red.